When it comes to global warming, hamburgers are the Hummers of food, scientists say. Simply switching from steak to salad could cut as much carbon as leaving the car at home a couple days a week. That's because beef is such an incredibly inefficient food to produce and cows release so much harmful methane into the atmosphere, according to Nathan Pelletier of Dalhousie University in Canada.

The livestock sector is estimated to account for 18 percent of global greenhouse gas emissions and beef is the biggest culprit.

A single kilogram of beef produces 16 kilograms carbon dioxide equivalent emissions: four times higher than pork and more than ten times as much as a kilogram of poultry.